Sucrose fatty acid esters exhibit excellent surface activity, satisfactory biological decomposability and high stability and have, therefore, been widely employed as additives for foods, cosmetics, pharmaceuticals, kitchen detergents, feeding stuff, resins, etc. or as assistants in the field of the chemical industry, for example, for polymerization reaction, oxidation reaction, and the like.
Extensive studies have hitherto been conducted on processes for preparing the sucrose fatty acid esters having such a wide variety of uses. Main processes so far developed comprise interesterification between sucrose and a lower alkanol ester or glyceride of a fatty acid (hereinafter referred to as a fatty acid ester) and are classified into the following three large groups.
Processes of the first group are included under a solvent process which comprises reacting sucrose and a fatty acid ester in a homogeneous system using a solvent capable of dissolving both the reactants, such as dimethylformamide, dimethyl sulfoxide, etc. According to this process, the reaction usually proceeds at low temperatures of about 90.degree. C. under reduced pressure. Catalysts which can be used include oxides, hydroxides, carbonates or hydrogencarbonates of alkali metals or alkaline earth metals, etc., with potassium carbonate being particularly preferred.
Processes of the second group comprise dissolving sucrose in a solvent, such as propylene glycol, water, etc., dispersing the resulting solution and a fatty acid ester with the aid of an emulsifier (e.g., fatty acid soaps) to form a very fine dispersion, i.e., a microemulsion, and removing the solvent to effect the interesterification reaction (microemulsion process). This reaction is usually carried out at a temperature between 110.degree. C. and 170.degree. C. under reduced pressure.
Processes of the third group comprise directly reacting sucrose and a fatty acid ester without using solvent (direct process). The reaction is usually carried out at a temperature of from 110.degree. C. to 140.degree. C. under normal pressure.
However, any of these conventional processes have their own disadvantages as set forth below and still leave room for further improvements.
The third process, i.e., direct process, enjoys advantages in that no solvent is required and that the reaction proceeds under normal pressure, but the problem here is how to mix the sucrose and the fatty acid ester which are incompatible with each other. The problem has been solved by adding a fatty acid soap to the reaction system or forming a fatty acid soap in situ. However, existence of a fatty acid soap in the reaction system means that an additional procedure for separating and removing the fatty acid soap would be necessary for recovery and purification of the desired product just as required in the microemulsion process.
The microemulsion process and the direct process employ reaction temperatures between 110.degree. C. and 170.degree. C., that are higher than those required in the solvent process (about 90.degree. C.), which results in considerable coloring of final products. In some extreme cases, the products should be subjected to a complicated decoloring treatment.
Considering hydropholic and lipophilic properties, that are significant characteristics of sucrose fatty acid esters, it is difficult to obtain sucrose fatty acid esters having a broad range of HLB value, a measure for hydrophilic and lipophilic properties commonly adopted in the art, by the microemulsion process or the direct process.
On the other hand, according to the solvent process, the products obtained are less colored as compared with those of the other processes owing to the relatively low reaction temperatures and may have a broadened range of HLB value. However, since the both reactants and the catalyst form a uniform solution in the presence of a solvent, it is necessary to add an acid to the reaction system for neutralization in order to stop the reaction so as to achieve desired reaction rate and degree of ester replacement. More specifically, in the case of using, for example, potassium carbonate as a catalyst, the reaction should be stopped by adding lactic acid, phosphoric acid or a solution containing the same, etc. to the reaction solution taking care on pH of the reaction solution Without this neutralization step, the reaction would further proceed due to the presence of the catalyst having interesterification activity even in the recovery and purification step of the product, which leads to reduction of yield of sucrose fatty acid esters having desired structure and composition. The neutralization step, in turn, brings about different problems, that is, water produced by the neutralization should be removed by a dehydration step, and the recovery and purification step becomes complicated because the reaction mixture contains a neutralization product, e.g., potassium lactate in the case of neutralizing a potassium carbonate catalyst with lactic acid or a lactic acid solution. Further, when the unreacted material, e.g., sucrose, is recylized to the reaction system, if potassium lactate is present in the recylized sucrose, the solubility of the potassium carbonate catalyst in the reaction solution is reduced to decrease the reaction rate.